For a number of years it has been known that various microorganisms produce metabolites exhibiting biological activity so as to be useful as biocides for combating diseases and pests.
Especially it has been known that some species of the genus Ascochyta produce different metabolites exhibiting biological activity, in particular as phytotoxins.
Examples thereof are e.g. an isoquinoline alkaloid, chrysantone, isolated from Ascochyta chrysanthemi (Phytochemistry, 28(3): 923-927, 1989), which is a phytotoxin, and which also exhibits an inhibitory effect on the growth of Bacillus cereus, B. subtilis, Escherichia coli and a weak antifungal activity on Cladosporium cucumerinum.
Chrysantone was found to have the formula (a) below: ##STR2## Ascotoxin (Decumbin) produced by A. imperfecta (Y. Suzuki, H. Tanake, H. Aoki and T. Tamura, Agric. Biol. Chem. 1970, 34, 395), is a strong plant pathogen with a phytotoxic activity exhibiting a strong inhibition of the growth of lucerne and rape. The isolated compound has the formula (b) below: ##STR3## Cytochalasin A and B produced by A. heteromorpha (J. Natural Products, 50 (5): 989-998, 1987) are well known myco- and phytotoxins belonging to the cytochalasins a class of mold metabolites exhibiting various effects. The cytochalasins are all characterized by a highly substituted isoindole ring to which is fused a macrocyclic ring. An example is cytochalasin B of formula (c) below: ##STR4## Solanopyrones A and C, two well known phytotoxins of formula (d) and (e), respectively, have been isolated from A. rabiei (Phytochem., 28: (10) 2627-2630, 1989) ##STR5## These compounds are not useful for combating plant diseases and pests because they are strong phytotoxins.
Although progress has been made in the field of identifying and developing biological biocides for combating various diseases and pests in plants and animals of agronomical importance, most of the biocides in use are still synthetic compounds that are difficult to decompose in nature and have a broad spectrum of activity.
In the last decade or so there has been a still increasing concern for the impact of such biocides on the environment and the ecosystems surrounding farmlands, and consequently there exists an outspoken need for biocides that are more specific in their activity, and are readily degradable in the natural environment.